Larger community and better support which means the platform is easier to sustain for sysadmins and more features to make it easier to sustain for mods. If Reddit is going to burn down for good, the community needs a forum platform more robust than what Flarum offers. It was best to make the platform switch while it wasn’t heavily used.
Flarum works and they’re improving their funding model so that Flarum can be better in the future, but they do not have anywhere near the dev community bandwidth to work on things like fediverse interoperability in a timely matter the way the Discourse community is:
As a point of reference of platform maturity, Discourse 1.0 released in 2014, Flarum 1.0, 2021.